Lectins, protein that bind carbohydrates, have been implicated as major determinants in Rhizobium-legume recognition. Lectins are also known to be involved in other biologically important examples of cell-cell recognition. The major goal of this research is to elucidate the molecular mechanism of host recognition in the Rhizobium japonicum-soybean symbiosis. The role of soybean lectins in this recognition will be critically examined. It is hoped that this research will add to the general knowledge of the mechanism of specific cell-cell recognition and the role of lectins in such recognition. The work will be greatly aided by a collection of mutants of R. japonicum defective in nodulation. A few of these mutants appear to be defective in attachment to soybean roots. Preliminary results with one of these mutants indicate that the defect lies in the lipopolysaccharide portion of the cell wall. The role of the LPS and other bacterial cell surface structures in plant attachment will be critically examined. Currently evidence for lectin involvement in Rhizobium-legume recognition is largely correlative and indirect. What is needed is an experimental system in which lectin binding produces a significant and reproducible change in nodulation. A few of the mutants of R. japonicum isolated demonstrate a defect in the ability to initiate infection. This defect can be phenotypically reversed by preincubation in soybean root exudate. Recent results indicate that lectin within the root exudate is responsible for the reversal in the phenotype of these mutants. The role of lectin in this reversal and the physiological effect of lectin binding to these mutants will be a major area of research focus. Previous work in this area has focused on the proposed role of lectins in attachment of Rhizobium to root surfaces. Our results indicate that lectins may function as signal molecules inducing specific physiological changes in the Rhizobium prior to attachment.